There have been numerous accidents resulting in severe injury or death where an explosive well tool such as a perforating gun fires prematurely at the surface while a crew was rigging the tool up in preparation for running it into the well. Various causes for such premature firing are set forth in the introductory portion of my U.S. Pat. No. 4,967,048, issued Oct. 30, 1990 which is incorporated herein by reference. The devices disclosed and claimed in the above-mentioned patent provide selectively operable ground connections that prevent sufficient electric current from flowing to the detonator to set it off until the ground connection is broken at the surface immediately before the tool is run into the well. Another approach to this problem is disclosed and claimed in my U.S. application Ser. No. 779,650 filed Oct. 21, 1991 which involves a ground connection that automatically is broken when the explosive tool has been lowered to a predetermined depth in the well. Still another approach is to provide a type of mechanical "safety" that prevents the tool from firing, even though an electrical signal of a sufficiently high current level is applied to the cable to set off the detonator, until the safety is released in some manner.
Various mechanical safety devices have been proposed, but to applicant's knowledge none have been particularly successful. For example, U.S. Pat. No. 4,523,650 discloses a rotatable plug that prevents firing until the plug has been turned by hand at the surface. U.S. Pat. No. 4,314,614 shows an interrupter or block that is attached to a lanyard which is pulled to remove the block prior to running the tool into the well. However, these devices arm the perforating gun while it is just below the rig floor, and if the gun prematurely fires at this location after these devices have been manipulated, well head equipment such as blow out preventers and the like will be perforated and rendered inoperable. Serious injuries still can occur, not to mention the delay, trouble and expense of replacing damaged equipment.
Another U.S. Pat. No. 4,266,613 shows a hydrostatic pressure operated electrical connector having a pin and a mating socket that are spaced apart until the tool reaches a certain depth in the well. However this device involves sequentially operated hot wire and ground switches, and is quite complicated and expensive to manufacture. Moreover the tool requires a large number of O-rings and other seals, which increases the likelihood of leakage and other operational problems that will cause improper operation.
An object of the present invention is to provide a new and improved hydro-mechanical safety device for a explosive well tool that obviates the above-mentioned problems.
Another object of the present invention is to provide a new and improved hydromechanical safety device of the type described which will not permit the explosive well tool to be fired unless it is below a predetermined depth in the well.